1. Field of the Invention
The present invention concerns novel (co)polymers and a novel radical polymerization process based on transition metal-mediated atom or group transfer polymerization (xe2x80x9catom transfer radical polymerizationxe2x80x9d).
2. Discussion of the Background
Living polymerization renders unique possibilities of preparing a multitude of polymers which are well-defined in terms of molecular dimension, polydispersity, topology, composition, functionalization and microstructure. Many living systems based on anionic, cationic and several other types of initiators have been developed over the past 40 years (see O. W. Webster, Science, 251, 887 (1991)).
However, in comparison to other living systems, living radical polymerization represented a poorly answered challenge prior to the present invention. It was difficult to control the molecular weight and the polydispersity to achieve a highly uniform product of desired structure by prior radical polymerization processes.
On the other hand, radical polymerization offers the advantages of being applicable to polymerization of a wide variety of commercially important monomers, many of which cannot be polymerized by other polymerization processes. Moreover, it is easier to make random copolymers by radical polymerization than by other (e.g., ionic) polymerization processes. Certain block copolymers cannot be made by other polymerization processes. Further, radical polymerization processes can be conducted in bulk, in solution, in suspension or in an emulsion, in contrast to other polymerization processes.
Thus, a need is strongly felt for a radical polymerization process which provides (co)polymers having a predetermined molecular weight, a narrow molecular weight distribution (low xe2x80x9cpolydispersityxe2x80x9d), various topologies and controlled, uniform structures.
Three approaches to preparation of controlled polymers in a xe2x80x9clivingxe2x80x9d radical process have been described (Greszta et al, Macromolecules, 27, 638 (1994)). The first approach involves the situation where growing radicals react reversibly with scavenging radicals to form covalent species. The second approach involves the situation where growing radicals react reversibly with covalent species to produce persistent radicals. The third approach involves the situation where growing radicals participate in a degenerative transfer reaction which regenerates the same type of radicals.
There are some patents and articles on living/controlled radical polymerization. Some of the best-controlled polymers obtained by xe2x80x9clivingxe2x80x9d radical polymerization are prepared with preformed alkoxyamines or are those prepared in situ (U.S. Pat. No. 4,581,429; Georges et al, Macromolecules, 26, 2987 (1993)). A Co-containing complex has been used to prepare xe2x80x9clivingxe2x80x9d polyacrylates (Wayland, B. B., Pszmik, G., Mukerjee, S. L., Fryd, M. J. Am. Chem. Soc., 116, 7943 (1994)). A xe2x80x9clivingxe2x80x9d poly(vinyl acetate) can be prepared using an Al(i-Bu)3: Bpy:TEMPO initiating system (Mardare et al, Macromolecules, 27, 645 (1994)). An initiating system based on benzoyl peroxide and chromium acetate has been used to conduct the controlled radical polymerization of methyl methacrylate and vinyl acetate (Lee et al, J. Chem. Soc. Trans. Faraday Soc. I, 74, 1726 (1978); Mardare et al, Polym. Prep. (ACS), 36(1) (1995)).
However, none of these xe2x80x9clivingxe2x80x9d polymerization systems include an atom transfer process based on a redox reaction with a transition metal compound.
One paper describes a redox iniferter system based on Ni(O) and benzyl halides. However, a very broad and bimodal molecular weight distribution was obtained, and the initiator efficiency based on benzyl halides used was less than 1% (T. Otsu, T. Tashingri, M. Yoshioka, Chem. Express 1990, 5(10), 801).
Another paper describes the polymerization of methyl methacrylate, initiated by CCl4 in the presence of RuCl2(PPh3)3. However, the reaction does not occur without methylaluminum bis(2,6-di-tert-butylphenoxide), added as an activator (see M. Kato, M. Kamigaito, M. Sawamoto, T. Higashimura, Macromolecules, 28, 1721 (1995)). This system is similar to the redox initiators developed early (Bamford, in Comprehensive Polymer Science (First Supplement), Allen, G., Aggarwal, S. L., Russo, S., eds., Pergamon: Oxford, 1991, vol. 3, p. 123), in which the small amount of initiating radicals were generated by redox reaction between (1) RCHX2 or RCX3 (where X=Br, Cl) and (2) Ni(O) and other transition metals. The reversible deactivation of initiating radicals by oxidized Ni is very slow in comparison with propagation, resulting in very low initiator efficiency and a very broad and bimodal molecular weight distribution.
Atom transfer radical addition, ATRA, is a well-known method for carbon-carbon bond formation in organic synthesis. (For reviews of atom transfer methods in organic synthesis, see (a) Curran, D. P. Synthesis, 1988, 489; (b) Curran, D. P. in Free Radicals in Synthesis and Biology, Minisci, F., ed., Kluwer: Dordrecht, 1989, p. 37; and (c) Curran, D. P. in Comprehensive Organic Synthesis, Trost, B. M., Fleming, I., eds., Pergamon; Oxford, 1991, Vol. 4, p. 715.) In a very broad class of ATRA, two types of atom transfer methods have been largely developed. One of them is known as atom abstraction or homolytic substitution (see (a) Curran et al, J. Org. Chem., 1989, 54, 3140; and (b) Curran et al, J. Am. Chem. Soc., 1994, 116, 4279), in which a univalent atom (typically a halogen) or a group (such as SPh or SePh) is transferred from a neutral molecule to a radical to form a new "sgr"-bond and a new radical in accordance with Scheme 1 below: 
In this respect, iodine atom and the SePh group were found to work very well, due to the presence of very weak Cxe2x80x94I and Cxe2x80x94SePh bonds towards the reactive radicals, (Curran et al, J. Org. Chem. and J. Am. Chem. Soc., supra). In earlier work, the present inventors have discovered that alkyl iodides may induce the degenerative transfer process in radical polymerization, leading to a controlled radical polymerization of several alkenes. This is consistent with the fact that alkyl iodides are outstanding iodine atom donors that can undergo a fast and reversible transfer in an initiation step and degenerative transfer in a propagation step (see Gaynor et al, Polym. Prep. (Am. Chem. Soc., Polym. Chem. Div.), 1995, 36(1), 467; Wang et al, Polym. Prep. (Am. Chem. Soc., Polym. Chem. Div.), 1995, 36(1), 465).
Another atom transfer method is promoted by a transition metal species (see (a) Bellus, D. Pure and Appl. Chem. 1985, 57, 1827; (b) Nagashima, H.; Ozaki, N.; Ishii, M.; Seki, K.; Washiyama, M.; Itoh, K. J. Org. Chem. 1993, 58, 464; (c) Udding, J. H.; Tuijp, K. J. M.; van Zanden, M. N. A.; Hiemstra, H.; Speckamp, W. N. J. Org. Chem. 1994, 59, 1993; (c) Seijas et al, Tetrahedron 1992, 48(9), 1637; (d) Nagashima, H.; Wakamatsu, H.; Ozaki, N.; Ishii, T.; Watanabe, M.; Tajima, T.; Itoh, K. J. Org. Chem. 1992, 57, 1682; (e) Hayes, T. K.; Villani, R.; Weinreb, S. M. J. Am. Chem. Soc. 1988, 110, 5533; (f) Hirao et al, Syn. Lett., 1990, 217; and (g) Hirao et al, J. Synth. Org. Chem. (Japan), 1994, 52(3), 197; (h) Iqbal, J; Bhatia, B.; Nayyar, N. K. Chem. Rev., 94, 519 (1994)). In these reactions, a catalytic amount of transition metal compound acts as a carrier of the halogen atom in a redox process, in accordance with FIG. 1.
Initially, the transition metal species, Mtn, abstracts halogen atom X from the organic halide, Rxe2x80x94X, to form the oxidized species, Mtn+1X, and the carbon-centered radical R*. In the subsequent step, the radical, R*, reacts with alkene, M, with the formation of the intermediate radical species, Rxe2x88x92M*. The reaction between Mtn+1X and Rxe2x80x94M results in the target product, Rxe2x80x94Mxe2x80x94X, and regenerates the reduced transition metal species, Mtn, which further reacts with Rxe2x88x92X and promotes a new redox process.
The high efficiency of transition metal-catalyzed atom transfer reactions in producing the target product, Rxe2x80x94Mxe2x80x94X, in good to excellent yields (often greater than 90%) may suggest that the presence of an Mtn/Mtn+1 cycle-based redox process can effectively compete with the bimolecular termination reactions between radicals (see Curran, Synthesis, in Free Radicals in Synthesis and Biology, and in Comprehensive Organic Synthesis, supra).
It is difficult to control the molecular weight and the polydispersity (molecular weight distribution) of polymers produced by radical polymerization. Thus, it is often difficult to achieve a highly uniform and well-defined product. It is also often difficult to control radical polymerization processes with the degree of certainty necessary in specialized applications, such as in the preparation of end functional polymers, block copolymers, star (co)polymers, etc. Further, although several initiating systems have been reported for xe2x80x9clivingxe2x80x9d/controlled polymerization, no general pathway or process for xe2x80x9clivingxe2x80x9d/controlled polymerization has been discovered.
Thus, a need is strongly felt for a radical polymerization process which provides (co)polymers having a predictable molecular weight and a narrow molecular weight distribution (low xe2x80x9cpolydispersityxe2x80x9d). A further need is strongly felt for a radical polymerization process which is sufficiently flexible to provide a wide variety of products, but which can be controlled to the degree necessary to provide highly uniform products with a controlled structure (i.e., controllable topology, composition, stereoregularity, etc.), many of which are suitable for highly specialized uses (such as thermoplastic elastomers, end-functional polymers for chain-extended polyurethanes, polyesters and polyamides, dispersants for polymer blends, etc.).
Accordingly, one object of the present invention is to provide a novel method for radical polymerization of alkenes based on atom transfer radical polymerization (ATRP), which provides a high degree of control over the polymerization process.
A further object of the present invention is to provide a novel method for radical polymerization of alkenes based on atom transfer radical polymerization (ATRP), which leads to more uniform and more highly controllable products (which are now obtainable only by living ionic polymerization methods).
A further object of the present invention is to provide a broad variety of novel (co)polymers having more uniform properties than those obtained by conventional radical polymerization.
These and other objects of the present invention, which will be readily understood in the context of the following detailed description of the preferred embodiments, have been provided in part by a novel process of atom (or group) radical transfer polymerization, which comprises the steps of:
polymerizing one or more radically polymerizable monomers in the presence of an initiator having a radically transferable atom or group, a transition metal compound and a ligand to form a (co)polymer, the transition metal compound having a formula Mtn+Xxe2x80x2n, and the ligand being any N-, O-, P- or S-containing compound which can coordinate in a "sgr"-bond or any carbon-containing compound which can coordinate in a xcfx80-bond to the transition metal, such that direct (i.e., covalent) bonds between the transition metal and growing polymer radicals are not formed, and
isolating the formed (co)polymer; and, in part, by novel (co)polymers prepared by atom (or group) radical transfer polymerization.